One of the ways in which a video signal may be degraded during processing in a TV studio or other production facility is by the addition of noise (random, short-term variations in amplitude) to the video signals. One of the major sources of noise in a production facility is a video tape recorder (VTR). When a video signal is recorded on video tape in a VTR, and is subsequently played back, the transformation of the picture information from electrical form to magnetic form and back again may result in addition of a significant amount of noise to the video signal, over and above any noise that may be added during processing of the picture information in electrical form. VTR manufacturers attempt to design and manufacture VTRs with the best possible signal-to-noise (S/N) ratio.
The standard method of measuring the S/N ratio for a component VTR involves measuring the noise level added to a component signal (luminance or chrominance, or R, G or B) at a region of the signal that is of constant amplitude. This practice has led some VTR manufacturers to incorporate so-called dead-zone circuits in the VTR at a point in the signal path that is downstream of the playback heads. A dead-zone circuit has the characteristic that it responds linearly to large changes in input signallevel but provides no response to small changes in input signal level. The threshold level, above which the circuit provides a linear response, is selected to be higher than the amplitude of the noise introduced in the record and play-back process. Consequently, the S/N ratio of the VTR, measured by the standard method, is very good. Use of a dead-zone circuit in a VTR might not be particularly objectionable in certain applications, e. g. where the VTR is used solely for recording and playing-back signals representative of pictures that have major areas of constant value and sharp transitions between areas of different value, such as many cartoons and certain computer generated graphics, but the dead-zone circuit results in serious degradation of signals representative of pictures with subtle gradations in value, such as pictures of human faces and of many indoor and outdoor scenes. In fact, after two or three tape-to-tape transfers, the dead-zone circuit can degrade a signal to the point that a serious question arises as to whether the signal can be used.
Dead-zone circuits are used with considerable subtlety. For example there may be different dead-zone circuits in high frequency and low frequency paths for the same component signal, and it is often difficult, even with the benefit of the schematic diagram for the VTR, to identify the dead-zone circuit and adjust it to minimize its deleterious effect.
The presence of a dead zone in a VTR can, in theory, be detected by use of a test signal comprising a horizontal sync pulse and a linear ramp. However, the dead zone only manifests itself by a slight shift in time of the ramp, and unless the original test signal and the signal received at the output of the VTR are superimposed on the screen of an oscilloscope or waveform monitor it is not possible to establish with a high degree of reliability whether there is a dead zone in the VTR.